I create covers for books which consist of 4 parts: a frontpage, a backpage, a middlepart (the spine of the book), and a Box creating margins arround the group of the other 3 parts. In order to achieve consistent results a use a 2-page documument with masterpage for front- and backpage and another 1-page document for the spine. I then mount those in a 3rd, oversized document or Artboard using the placement-option (linking to the 2 files for front/back and spine). After adjusting them so they form the correct shape, I group these 3 objects and finally add a layer beneath the group with a larger rectangle filled in the same color as the rest of the files (creating margings around the grouped objects. When exporting to pdf my printing-house tells me, that there are color-differences between the embedded pages and the margin-rectangle and when checking this in pdf-XChange I do also see different colors (unfortunately it only displays RGB-Values, but all of the files are in CMYK/8). Opening the same pdf in affinity designers DOES not show any difference between the different parts.
I am on windows10 machines and have reproduced this problem in Affinity 2.3, 2.4 and even in the latest version of 1. All documents are in CMYK/8 with "coated FOGRA-39" profile applied (pdf-Export also with CMYK/8 and ICC-profile "coated FOGRA-39" enabled. Problem persists if switching to designer to mount the parts.
In order to illustrate the problem I created a simplified version and attach 3 files: the file containing the spine (middle.afpub), the file containing the a link to the spine file (right side) and a copy of the layoutpage from the spine file (using clipboard) on the left. The 3rd file is the exported pdf.
Comparing the rgb-values with a direct pdf-export from the spine file (not attached here) shows the same values as the clipboard-copy, implying that the problem lies with the linking. Unfortunately I have no software available to check the CMYK-values within the pdf, but I suspect, that the linked part (right side) no longer shows the values used in constructing (5/100/80/0).
Any ideas whether this is a bug or whether I am doing something wrong are very welcome...
Simpletest.afpub
Simpletest.pdf
Middle.afpub